The disclosure of U.S. Provisional Application No. 61/527,440, filed Aug. 25, 2011, is hereby incorporated herein in its entirety by reference.
Continuously variable transmissions utilizing a hydrostatic power unit (HSU), hereinafter sometimes referred to as hydro-mechanical continuously variable transmissions, are well known. A variety of work machines utilize this type of transmission for industries such as agriculture, earth moving, construction, forestry, and mining. In operation, the fluid displacement of the hydrostatic power unit is varied to change the output to input ratio of the transmission, that is, the ratio between the rotating output of the transmission, and the input. This is accomplished by varying the angle in a swash plate of a variable displacement fluid pump or motor of the hydrostatic unit. In one category of continuously variable transmissions, the hydrostatic power unit is configured such that at zero vehicle or machine speed, the swash plate of the hydrostatic power unit is at full displacement or near full displacement, in one direction or the other, depending on the range selected, direction of travel and possibly other factors. Reference as an example in this regard, Weeramantry, U.S. Pat. No. 7,063,638 B2, issued Jun. 20, 2006.
Essentially, in operation, the operator commands desired vehicle speed in some manner, for instance, utilizing an input device such as a propulsion handle. This command is received by the transmission controller, and is translated into a desired speed ratio for the transmission, which is defined as the output speed divided by the engine speed, denoted by the term transmission reciprocal ratio or “TRR”. Working in the TRR has several advantages. It is more directly related to the ultimate control output (swash plate angle) than the vehicle speed. Also, momentary changes in engine speed don't change the target TRR, and don't affect the swash plate angle, so as to avoid unnecessary changes to the output.
The desired TRR. or “DTRR” is converted into the desired hydrostatic power unit reciprocal ratio or “DHRR”, which is defined as the hydrostatic motor speed divided by the pump speed (the hydrostatic reciprocal ratio being referred to herein generally as “HRR”). This is done using knowledge of the layout and gear ratios of the transmission, current speed range, and direction, using equations for the particular transmission. The heart of the controller is closed loop control of HRR. The HRR quantity is chosen since it directly relates to the swash plate angle which is the output of the controller, HRR can be directly calculated from the engine speed sensor and the motor speed sensor, using known gear ratios, Use of HRR is also superior to other methods of control, such as adjusting the swash plate directly for ground speed errors or ratio errors, since it results in the same controller performance in all speed ranges. Typically, the closed control loop for HRR uses a proportional, integral and optionally differential term or “PID”.
However, it has been observed that closed loop control can suffer from shortcomings and disadvantages including oscillations and slow response. As an example, a closed loop control will have to determine and apply a different control signal value for controlling the swash plate angle to maintain the motor speed before and after a range shift. Usually, the load on a tractor remains about constant before and after the shift, requiring the same torque, but the relationship between torque and HSU pressure is different in each range, causing a disturbance to the PID loop. As a range shift occurs, the HSU moves from regeneration to non-regeneration, and the pressure changes, causing the disturbance.
Thus, what is sought is a manner of controlling swash plate angle in a hydrostatic power unit of a continuously variable transmission, which overcomes one or more of the disadvantages or shortcomings, and achieving one or more of the desired characteristics, set forth above.